#!/bin/bash
# daysuntil -- Basically this is the daysago script backwards, where the
#     desired date is set as the current date and the current date is used as
#     the basis of the daysago calculation. 

# Like the previous script, use `which gdate` if you are on OS X
# If you are on Linux, use `which date`
setvar date = "$(which date)"

proc daysInMonth {
  case (1) {
    1|3|5|7|8|10|12  { setvar dim = '31' } # most common value
    4|6|9|11         { setvar dim = '30' }
    2                { setvar dim = '29' }  # depending on if it's a leap year
    *                { setvar dim = '-1' }  # unknown month
  }
}

proc isleap {
  # If specified year is a leap year, returns non-zero value for $leapyear

    setvar leapyear = $($date -d 12/31/$1 +%j | grep 366)
}

#######################
#### MAIN BLOCK

if test $Argc -ne 3  {
  echo "Usage: $(basename $0) mon day year"
  echo "  with just numerical values (ex: 1 1 2020)"
  exit 1
}

$date --version > /dev/null 2>&1         # discard error, if any

if test $? -ne 0  {
  echo "Sorry, but $(basename $0) can't run without GNU date." >&2
  exit 1
}

eval $($date "+thismon=%m;thisday=%d;thisyear=%Y;dayofyear=%j")

setvar endmon = "$1"; setvar endday = "$2"; setvar endyear = "$3"

# lots of parameter checks needed...

daysInMonth $endmon	# sets $dim variable
if test $endday -lt 0 -o $endday -gt $dim  {
  echo "Invalid: Month #$endmon only has $dim days." >&2 ; exit 1
}

if test $endmon -eq 2 -a $endday -eq 29  {
  isleap $endyear
  if test -z $leapyear  {
    echo "Invalid: $endyear wasn't a leapyear, so February only had 28 days." >&2
    exit 1
  }
}

if test $endyear -lt $thisyear  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current year." >&2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -lt $thismon  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current month." >&2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -lt $thisday  {
  echo "Invalid: $endmon/$endday/$endyear is prior to the current date." >&2 ; exit 1
}

if test $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -eq $thisday  {
  echo "There are zero days between $endmon/$endday/$endyear and today." >&2 ; exit 0
}

## if we're working with the same year, the calculation's a bit different

if test $endyear -eq $thisyear  {

  setvar totaldays = $(( $($date -d "$endmon/$endday/$endyear" +%j) - $($date +%j) ))
  
} else {

  ## calculate this in chunks: starting with days left in this year

  ###  DAYS LEFT IN START YEAR

  # calculate the date string format for the specified starting date

  setvar thisdatefmt = ""$thismon/$thisday/$thisyear""

  setvar calculate = ""$($date -d "12/31/$thisyear" +%j) - $($date -d $thisdatefmt +%j)""

  setvar daysleftinyear = $(( $calculate ))

  ###  DAYS IN INTERVENING YEARS

  setvar daysbetweenyears = '0'
  setvar tempyear = $(( $thisyear + 1 ))

  while test $tempyear -lt $endyear {
    setvar daysbetweenyears = $(( $daysbetweenyears + $($date -d "12/31/$tempyear" +%j) ))
    setvar tempyear = $(( $tempyear + 1 ))
  }

  ###   DAYS IN END YEAR

  setvar dayofyear = $($date --date $endmon/$endday/$endyear +%j)	# that's easy!

  ###   NOW ADD IT ALL UP

  setvar totaldays = $(( $daysleftinyear + $daysbetweenyears + $dayofyear ))
}

echo "There are $totaldays days until the date $endmon/$endday/$endyear."
exit 0